1. Field of the Invention
The present invention relates to a peptide capable of inducing CTL (Cytotoxic T Lymphocytes; refer to Medical Immunology, revised 3rd edition, compiled by Kikuchi Kokichi) to human gastric cells in vivo or in vitro, and to a DNA encoding this peptide. More specifically, the present invention relates to a peptide capable of presenting CTL to human gastric cells by being bound to HLA-A31 antigen (Human Leucocyte Antigen; refer to Modern Immunology, 2nd edition, compiled by Yamamura Yuichi and Tada Tomio), and to a DNA encoding the peptide.
The present invention further relates to an agent useful for preventing or treating human gastric cancer, the agent containing a peptide capable of inducing CTL to a human gastric cancer cell in vivo or in vitro, and to a vaccine for preventing or treating human gastric cancer, the vaccine containing a recombinant virus or a recombinant bacterium having a DNA encoding such a peptide.
2. Discussion of the Background
For pharmacotherapy of malignant tumors, a chemotherapeutic agent which directly impairs tumor cells or an immunotherapeutic agent with which treatment is conducted by non-specifically activating an immunity of a host and enhancing a bioprotective function of the host has been used. However, there is currently no agent with which malignant tumors, above all, tumors of the digestive tract can completely be cured.
In recent years, researchers using tumors of animals, mainly mice, have revealed that tumors can completely be cured by enhancing an antigen-specific immune response to tumor-related antigens and tumor-specific antigens present in various tumor cells. Clinically, the treatment has been conducted by enhancing the antigen-specific immune response to these tumor-specific antigens.
With respect to agents or approaches by which tumors are treated upon enhancing the tumor antigen-specific immune response, it has been reported that a monoclonal antibody against an antigen which is expressed in tumor cells and mainly recognized by B cells is used, that a tumor-specific immune response is induced by administering to the patients their own tumor cells or solubilized fractions thereof which have been inactivated with radiation or medicines as a vaccine, and that in order to increase an immunogenicity of tumor cells, viruses or various cytokine genes are introduced into tumor cells and the thus-treated tumor cells are administered to the patients themselves, whereby a tumor-specific immune response is induced.
However, it is being made clear that T cells including CTL mainly act on tumor rejection in vivo. It has been currently clarified that the treatment using an antibody against a B-cell-recognition antigen is limited.
Further, it has been clarified that the immune response with the T cells acts either intensively or suppressively in a state where two functional sub-populations (Th1 and Th2 subsets) present in T cell populations are activated, and that the immune of tumor cells themselves containing plural antigens sometimes rather suppresses the immune response to the tumor cells.
Still further, with respect to the inoculation of inactivated tumor cells, the possibility of re-growth of tumors through re-activation in vivo cannot completely be denied. Thus, the problem of safety remains.
In order to sidestep the above-mentioned problems, a method is considered in which an antigen protein to activate CTL which is deemed to play a central role in tumor rejection is identified and CTL is activated using this antigen protein.
It has been clarified through advances of researches in recent years that CTL is induced such that a peptide which is fragmented by a protease in cells and which comprises from 8 to 12 antigen-protein-derived amino acids acts as an antigen presenting molecule to CTL by being bound to an HLA antigen.
Accordingly, regarding the cancer antigen peptide, it is considered that if a peptide which is bound to an HLA molecule to induce CTL to tumor cells can be discovered, this peptide can he used as an agent for preventing or treating cancers.
On the basis of these considerations, a cancer antigen peptide capable of inducing CTL has been studied. However, only a peptide derived from a protein, i.e. MAGE family, Mart-1, Tyrosinase, gp100, which is a cancer antigen present in melanoma tumors has been made clear to date. With respect to cancers of the digestive tract including gastric cancer, the presence of a cancer antigen peptide capable of inducing CTL and the structure of the cancer antigen peptide are currently unknown.
Accordingly, it is one object of the present invention to provide novel peptides capable of inducing an immune response to human gastric cancer.
It is another object of the present invention to provide novel DNA encoding such a peptide.
It is another object of the present invention to provide novel agents or compositions useful for treating or preventing human gastric cancer, which contain such a peptide.
It is another object of the present invention to provide novel recombinant viruses and novel recombinant bacteria which contain DNA encoding such a peptide.
If is another object of the present invention to provide novel vaccines useful for preventing human gastric cancer, which contain such a recombinant virus or recombinant bacterium containing DNA encoding such a peptide.
If is another object of the present invention to provide a novel method for preparing such a peptide by culturing such a recombinant virus or recombinant bacteria.
To achieve these objects, the present inventors have focused on the fact that CTL which recognizes the tumor antigen plays an important role as a bioprotective mechanism to tumor cells. That is, they have conducted studies in consideration of the fact that the efficient induction of CTL with a peptide which can be used as a part of a vaccine or with a transformant containing a DNA encoding the peptide is effective for preventing or treating cancers.
It has been known that the HLA antigen is bound to the cancer antigen peptide and acts as an antigen-presenting molecule to CTL whereby CTL is induced. Accordingly, it is considered that if a peptide which can activate CTL reactive with gastric cancer cells can be found, it is possible to use the same as an agent for preventing or treating the gastric cancer.
The present inventors have succeeded in establishing a CTL cell strain (Tc-HST-2) which is specifically reacted with gastric cancer cells restrained to HLA-A31, this strain being derived from the patient suffering from a gastric cancer, and they have succeeded in establishing a gastric cancer cell strain (HST-2) on which this CTL acts, this strain being derived from the same patient (J. I. Meth., vol. 154, pp. 235-243 (1992) and Cancer, vol. 75, pp. 1484-1489 (1995)).
However, it has been unclear whether this CTL recognizes the tumor antigen and what antigen this CTL recognizes. On top of that, the presence of the cancer antigen peptide which exists specifically in cancers of the digestive tract including gastric cancer and which is capable of inducing CTL has not been clarified at all; much less has the essence thereof been clarified.
Accordingly, the present inventors have purified an HLA-bound peptide present on the cell surface of gastric cancer cell HST-2, and have identified a peptide that activates the CTL cell strain Tc-HST-2. Further, they have chemically synthesized this identified peptide in a conventional manner. Thus, it has been clarified for the first time that this peptide actually activates the CTL cell strain Tc-HST-2. Still further, it has been found that this peptide is expressed in gastric cancer cells other than HST-2.
From these results, it has been clarified that the above-mentioned peptide can be utilized well as an agent for preventing or treating gastric cancer. These findings have led to the completion of the present invention.
Thus, the present invention provides: (1) a peptide which is a fragment of a gastric cancer antigen protein present in a human gastric cancer cell, the fragment being bound to an HLA molecule and capable of inducing a cytotoxic T cell that targets the gastric cancer cell; (2) an agent for preventing or treating human gastric cancer, the agent containing the above-mentioned peptide; (3) a DNA encoding the above-mentioned peptide; and (4) a vaccine for preventing or treating human gastric cancer, the vaccine containing a recombinant virus or a recombinant bacterium having this DNA.